Introduction to NMR Spectroscopy - PowerPoint PPT Presentation

Introduction to NMR Spectroscopy. Part I. Introduction. Nuclear Magnetic Resonance (NMR) Spectroscopy is a technique that is used to determine the type, number and relative positions of certain atoms in a molecule

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Nuclear Magnetic Resonance (NMR) Spectroscopy is a technique that is used to determine the type, number and relative positions of certain atoms in a molecule

Originally discovered by Felix Bloch and Edward Purcell in 1946 (both shared the Nobel Prize in Physics in 1952 for their pioneering work), it has seen a significant increase in popularity with the development of FT-NMR spectrometers

Most elements possess at least one NMR active nucleus, but many of them several (i.e.,115Sn,117Sn and 119Sn, 95Mo and 97Mo, etc.). In order for an atom to be NMR active, the spin quantum number (I) must not equal zero.

If the proton and neutron number are even, the spin quantum number will be zero. Both 12C and 16O will not be observable, but 13C, 1H and 17O are NMR active nuclei.

Nuclei with a spin quantum number larger than I=½ often show broad lines because of their quadrupole moment

There is a significant difference in abundance in these NMR active nuclei and the sensitivity of these experiments differs quite a bit as well.

From coupled spectra, it is possible to obtain information about the neighboring atoms based on the splitting of the signal

This holds especially true for proton spectra, where the multiplet structure reveals how many hydrogen neighbors a given CHx-function (x=1-3) has

Most of the 13C-NMR spectra are obtained as proton-decoupled spectra (13C{1H}), which means that this information cannot be obtained from those spectra. However, the coupling with other nuclei (i.e., D, F, P, etc.) will still be observed (i.e., CDCl3 display a triplet at d=77 ppm)

The chemical shift of protons is mainly due to the effect of neighboring groups, which are either electron-withdrawing groups/atoms that cause protons to be more deshielded, or electron-donating groups/atoms, which results in more shielded protons.

The first group causes a shift downfield (to higher ppm values!), while the second group causes the signals to appear upfield (at lower ppm values). Several effects influence these shifts.

Electronegativity (red line in graphs on the right is d=3 ppm)

The higher the electronegativity of the attached heteroatom, the further downfield the corresponding signal is shifted due to the deshielding of the hydrogen atom. Note that the effect is fairly pronounced in some cases because hydrogen is less electronegative compared to carbon (EN=2.5).

In arenes, alkenes, alkynes and for carbonyl functions a special effect is observed, called anisotropy

These functional groups possess circulating p-electrons, which cause a secondary magnetic field

The chemical shift of the protons in these molecules highly depends where these protons are located in respect to this secondary magnetic field. (“+” denotes shielded areas, while “-“ denotes deshielded areas)

In the case of arenes, alkenes and carbonyl functions these protons exhibit less shielding and are shifted downfield

In alkynes, the protons are located in the area of increased shielding and therefore are less shifted than alkene protons

The NMR spectroscopy cannot only distinguish between magnetically different protons, but also determine the approximate ratio of these protons

The NMR spectrometer does the integration and provides the information either as a number under the signal as shown in the spectrum below (39.9 and 60.0) or draws a vertically rising line

In order to determine the true ratio of the signals, the distance between the foot and the top of the integration line above a peak has to be measured

All values are then divided by the smallest number to obtain the relative ratios. If a ratio is not an integer (i.e., 1:1.5), a factor has to be found to make it an integer as shown in the example above (multiply by 2 makes it 2:3)

The multiplet structure of a signal is due to a spin-spin splitting of magnetically non-equivalent protons. For a group of n adjacent protons, a signal containing (2*n*I+1=2*n*½= n+1 for I=½) peaks is observed.

For instance, bromoethane exhibits a triplet (=three peaks) at d=1.53 ppm for the methyl group (CH3) due to the splitting from the two neighboring hydrogen atoms.

The methylene group (CH2) shows as a quartet (=four peaks) at d=3.31 ppm, which is shifted downfield because of the bromine attached to the same carbon atom.

There is no splitting observed within the methyl or methylene group here because there is a free rotation about the C-C single bond making all protons within these groups chemically equivalent.

The distance between the individual peaks of a multiplet is called spin coupling constant (J).

These protons can have different spins (mI= ±½) and therefore cause an additional shielding (same spin compared to the applied field) or deshielding (opposite spin) of the observed protons. If there are more than one hydrogen atom on the adjacent C-atom, more spin arrangements will be possible i.e., methyl group.

The methylene group will appear as a quartet. The four lines will display a relative intensity of 1:3:3:1 (theoretically).

A neighboring methyl group splits a signal into a quartet, which ideally shows relative intensities of the peaks of 1:3:3:1. Generally, the line intensities can be predicted using Pascal’s triangle (for well separated multiplets using nCr):

1Singlet

1 1 Doublet

1 2 1 Triplet

1 3 3 1 Quartet

1 4 6 4 1 Quintet

1 5 10 10 5 1 Sextet

1 6 15 20 15 6 1 Septet

17 21 35 35 21 7 1 Octet

1 8 28 56 70 56 28 8 1 Nonet

The higher the multiplicity, the smaller the outer lines are compared to the next line

In cases, when a lot of lines are observed, it is difficult to identify the exact number of lines within a multiplet because the outermost lines are barely (or not) visible in those cases

Sometimes it helps to determine the ratio of the two lines farthest to the outside of the multiplet.

If the coupling multiplets are close together, the ratio of the intensity of the lines changes. This effect is called multiplet skewing (“leaning”) and allows one to locate the coupling partner.

The outermost lines tend to be smaller than the innermost lines of a coupling system as the following scheme.

This effect is the greater the closer the signals are. This can even lead to the disappearance of the outermost lines i.e., in the aromatic range because the signals are relatively close together there. In some cases a triplet converts into a ‘doublet’ or two doublets appear as one ‘singlet’ due to this effect.

The situation is more complicated in aromatic systems, which often show very complicated (due to overlap and long-range coupling through the p-system) and difficult to analyze patterns for beginners.

The following examples illustrate some important points but are by all means far from being complete.

The first step is to understand the patterns in the aromatic range due to symmetry, the second step is to identify the effect of different groups onto the various protons on the ring.

Aromatic protons usually show up in the range of d=6-9 ppm (Strictly speaking, the coupling patterns are much more complicated, but for the sake of simplicity only first order couplings will be analyzed here because this is what can be observed on a normal spectrum!)

The two signal groups at d=7.4-7.5 ppm corresponds to the ring protons, while the singlet at d=2.6 ppm is due to the methyl group on the ring.

The expansion of the aromatic range on the right hand side shows a triplet (Hm) and a triplet (Hp) that is overlapped by a doublet (Ho) on the left side. The ortho and para protons are shifted about the same if a methyl group is attached to the ring (Dd = -0.18 ppm (ortho) and Dd= -0.20 ppm (para)).

In addition, a strong multiplet skewing is observed because the signals are very close together. Note that the two outer lines of the triplet at d=7.5 ppm possess very different intensities.

If the substituent R was an electron-donating group i.e., alkoxy (i.e., anisole), amino (i.e.,aniline), a distinguished splitting of the protons would be observed in this region of the spectrum.

The meta protons are slightly shifted downfield (triplet at d=7.48 ppm), while the ortho (doublet at d=7.12 ppm) and para protons (triplet at d=7.15 ppm) are shifted upfield, because the electron-density increased in these positions (as shown in the diagram).

The singlet around d=3.9 ppm is due to the methyl group that is attached to the oxygen atom.

The triplet at d=7.66 ppm is due to the meta protons, while the doublet for the ortho and para proton overlaps d=7.1-7.2 ppm.

The methyl groups are less shifted (d=3.2 ppm) due to the lower electronegativity of the nitrogen atom as compared to the oxygen atom, but the integration for this signal is higher because it represents six equivalent hydrogen atoms.

The signal at d=8.0 ppm is due to ortho hydrogen atoms (downfield shift ~0.65 ppm), while the signal at d=7.2-7.4 ppm is due to the meta and para hydrogen atoms (both triplets downfield shift about 0.1-0.2 ppm).

The quartet at d=4.3 ppm corresponds to the CH2-group in the ester part. The increased shift is due to the oxygen atom of the ester function. The triplet at d=1.35 ppm is due to the methyl group.

This substitution pattern will usually lead to a symmetric set of signals, consisting of a doublet (H1) and a “triplet” (H2),both with an integral of two hydrogen atoms.

Often times, these signals are very close together and/or overlap. However, the signal groups are usually relatively symmetric.

Case 2: two different substituents

An asymmetric ortho-substitution leads to a very complex splitting pattern in the aromatic range because there is no symmetry anymore (H1 and H4 form a doublet each, H2 and H3 form a triplet each, integration one hydrogen atom each).

Due to the possible overlap, these patterns are often difficult to recognize and analyze as well.

If both substituents are the identical, a symmetry plane (going through C1 and C4) will be observed in the molecule.

As a result three signals are observed: a singlet (H1), a doublet (H2) and a triplet (H3) (integration ratio: 1 H:2 H:1 H).

Due to the possible overlap, these patterns are often difficult to recognize and analyze as well.

Case 2: two different substituents

An asymmetric meta-substitution leads to a very complex splitting pattern in the aromatic range: H1 forms a singlet, H2 and H4 show as a doublet each, and H3 as a triplet (integration one hydrogen each).

Due to the possible overlap, these patterns are often difficult to recognize and analyze as well.

The spacing between the lines of a multiplet is called coupling constant.

The coupling constant is identical within the multiplet and its coupling partner. In other words, nucleus A couples with nucleus B with the coupling constant JAB, and nucleus B couples with nucleus A with the same coupling constant, JAB. This allows matching multiplets, which couple with each other.

Coupling constants are angle dependent as can be seen in the in the diagram below, which was generated using the vicinal Karplusrelationship (M. Karplus, Noble Prize in Chemistry in 2013).

The highest J-values are obtained for angles of Q=0 and 180o, while the J-value for Q=90o is very small.

The degree of coupling is a function of the overlap of the involved orbitals. If they are co-aligned, the interaction will be very strong. If they are perpendicular, the overlap is going to be weak resulting in a low coupling constant.

Coupling constants are usually given in Hertz (Hz) and not in ppm. For proton spectra they are usually in the range of JH-H=0-20 Hz (see below),while the coupling constants with other nuclei are often significantly larger (~102-103 Hz) i.e., JH-F(CH2F2)= 50 Hz, JP-H((CH3)2PH)=192 Hz, etc. Coupling constants are independent from sweep frequency of the NMR spectrometer used.